OPTICAL SENSE-ORGANS OF LEAVES 615 



characteristic of epidermal cells that are concerned with the perception 

 of stimuli. 



Kohl asserts, that in negatively heliotropic roots the faculty of light- 

 perception is confined to the tip. 317 There is no evidence, however, 

 that sensitiveness is more strictly localised in the case of these organs, 

 nor is a specialisation of this nature to be expected on theoretical 

 grounds. 



B. THE OPTICAL SENSE-ORGANS OF FOLIAGE-LEAVES. 318 



A great many foliage-leaves are diaheliotropic ; as Wiesner 319 has 

 shown, their blades are in a condition of heliotropic equilibrium, when 

 they are placed at right angles to the direction of the most intense 

 diffuse illumination to which they are ordinarily exposed. By assuming 

 this " heliotropically fixed position " {fixe IAchtlage) the leaf is assured 

 of the maximum amount of illumination. Leaves which behave in this 

 manner are termed " euphotometric" by "Wiesner; they are particularly 

 prevalent among shade-plants. 



Leaves assume their heliotropically fixed position by means of appro- 

 priate curvatures or torsions; the motor-organ may consist of the whole 

 petiole or of a pulvinoid region thereof, or sometimes of a typically 

 developed pulvinus. Dutrochet long ago suggested, with reference to 

 these cases of euphotometric adjustment, that the leaf-blade exerts a 

 directive influence upon the actual motor-organ represented by the 

 petiole. Vochting 320 was, however, the first to prove experimentally, in 

 in the case of Malva verticillata, that the light-perceiving lamina deter- 

 mines the behaviour of the petiole, forcing the latter, if need be, to 

 execute movements in opposition to its own heliotropic tendencies, in order 

 to bring the leaf-blade into a favourable relation to the available light. 

 The author has since succeeded in demonstrating a similar directive influ- 

 ence of the lamina in several other plants. Thus, the leaf-blade of Begonia 

 discolor assumes the fixed position, even if the petiole is completely 

 darkened by means of a sheath of tinfoil. A similar result is obtained 

 with Monstera deliciosa, if the distal region of the petiole, which is 

 developed as a pulvinus, is covered with tinfoil ; although itself 

 shaded, the pulvinus executes the curvature or torsion necessary to 

 restore the lamina to the position of heliotropic equilibrium, with the 

 greatest possible precision. In species of Tropaeolum (T. majus, T. minus, 

 T. Lobbianum) the results of similar experiments lead one to conclude 

 that it is only the coarse adjustment of the leaf-blade that is brought about 

 directly by the positive heliotropism of the petiole, the fine adjustment 

 being effected indirectly, through the influence of the lamina upon the 

 petiole. The primary leaves of Phaseolus multifiorus, on the other hand, 

 take up their fixed position, as Krabbe 321 had already remarked, even 



